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Wednesday, May 18, 2011

Board #2 Debrief

Before it starts to fads from memory I wanted to run through for my own learning what worked and what didn't with board #2.

Core preparation

The good

i) inserts: using the flange nuts pushed through ABS washers with the washers having 1mm holes drilled through it were a great replacement for the t-nuts. The small holes let the air and resin flow through to the top so the insert can be moved easily to the right right spot. I drilled the insert holes all the way through the board this time and filled the bottom with about 3mm of 5 min epoxy. When I set the inserts in place I put a bolt all the way through the insert and sat it on top of the 5 min epoxy. In board #1 there wasn't the 3mm layer before the laminate and so when I overtightened the screw it pushed through to the fibreglass and delaminated it.

Also, pouring the resin very slowly into the insert hole let the resin push the air out ahead of it so I had much less air trapped in the resin.

ii) Using a really thick q-cell (microballons) mix to put a radius on the join between the upper layer and the lower layer. The mix was really like peanut butter. I had used much thinner mix in the board #1 and a lot of it flowed away and I had to keep reworking until it set.

iii) rails: Cutting the ABS in straight strips (10mm) and then heating and forming it around the shape of the core worked very well. Last time I tried to cut the ABS to the shape of the board and wasted a lot of material and didn't get a good mating of the rails and core.

iv) Not gluing the long sides of the balsa veneer together. This lead to needing less clamping pressure to get the concave in and as the gaps open very slightly when the concave was added it allowed the resin to get right into the groove.

v) Sanding: I found that sanding with sandpaper tapped to long planks of wood helped get long smooth curves. I found that small sanding block lead to the board edge being a bit wavy.

The bad

Air bubbles
I still struggled to keep air bubbles out of the q-cell / resin mix. I had tried to mix it more slowly as I'd read that vigour mixing can put air in the mix that ends up getting trapped. The bubble issure reduced but was still more like an aerobar that I wanted. I primarily used it in the inserts. I used q-cell here to reduce the weight and to ensure the inserts themselves where not visible. i think that next time I'll just use pigment to obsure inserts. I don't think that the weight saving is material enough to warrant putting the strength at risk.

Gluing the rails to the core - The low viscosity epoxy resin easily flowed away from the gaps created by the slight mis-match between the core and the rail. The heated forming of the rails meant the mis-match was small and I used some q-cell in to thicken it up and improve its vertical hold. Still it flowed away and pooled under the board. Solution would be to put tape on the underside and rail to hold it in place and create prevent the resin flowing away (der!) Pouring the rails seems to all round be a better solution - no adhesion issues no shaping issues.

Laminating flat - with the balsa wood being stiffer, the final core was significantly stiffer that than the klegecell alone. I suspect this may have lead to some more spring back in the final board than board #1. Laminating it all on the rocker table might have been a better idea so that some of the shape is all ready held in.

Balsa veneer - the balsa veneer looks great and I'm going to make more this way - it just looks very specy. However, laminating with just a thin (1mm) veneer created more work than was probably necessary as I needed to glue the spars around the outside of the top layer to that the foam core wasnt' visible. This meant a long wait time again before proceeding. I think that next time I might use 5mm balsa as the top layer rather than 2 layers of foam and balsa veneer. This will speed the process up but will make for a slightly heavier baord.

Rocker table

Go the perspex!!! Straight off the table the bottom finish was awsome. I didn't even need to clear coat it (but probably will for protection) to get a mirror finish. Also, tacky tap for vac bagging had no problems sealing to it. One tip with tacky tap - when removing the vac film rip it of in one hard quick motion. It will come off without stretching the vac film or tape so it can be reused. Didn't need much mould release to get the resin to separate from it. I suspect you may not need any but why run the risk.

Board-off Design Tool. Being able to print the design templates for the rocker table meant I got the rocker and concave spot on.

Sanding the surfaces of the side jigs on the rocker table at an angle so that the rocker surface when it bends down to sit between then the surfaces of the jigs are parallel to the rocker table surface. If the aren't flush with the the bent surface the amount of rocker and concave clamped into the surface will be wrong as there will be a significant gap between the top and bottom jigs.

Laminating

The good

i) Mixing the right amount of resin (according to the datasheets) so that you don't end up using too much resin. Not only does it leave a mess on the rocker table and the board but it will weaken the board if excess resin remains. This is because more of the internal stresses will be borne by the resin (as there is more of it) and resin is about 30 times weaker than e-glass. Hence it breaks sooner. I followed the 50:50 ratio by weight and added an extra 10% for resin left in the cup and excess need to ensure it saturated well. I wasted so little resin this time and using the right amount more than likely reduced the potential for resin pooling under the peel ply.

ii) Use proper peel ply - in board #1 I used ripstop nylon left over from an old kite. I put a lot of holes in it to help the resin flow through it to the bleeder material and thought that the weave of the material would be enough to avoid resin pooling under it. However, the combination of too much resin and the nylon having too little porosity meant the resin pooled under the nylon and left the surface of the board very bumpy. Presumably I could have sanded it back but that seemed like a lot of hard work.

This time I used proper peel ply which wicks the excess resin away and apart from the imprint of the weave left an very uniform surface - no big lumps. Its cheap about $5 a meter. Well worth it.

iii) Used proper vac film - I've been told that the benefit of vac film is that it stretched around sharp corners. I fyou have a relatively flat piece you're laminating then you can use standard old builder plastic that you can buy from bunnings. I found some builder plastic but it was only marginally cheaper and had grit embedded in it. Not sure if there is any advantage to it. The vac film, which is polyethylene plastic is strong and uniform so I can see any good reason to substitute.

The bad

i)_Air bubbles in the laminate. I am starting to believe that some air bubbles are inevitable and require some post cure work to cover and fill them. I worked the resin a hell of a lot harder with the squeegee this time but the tiny air bubbles trapped in the bottom layer only reduced a little bit. I have been told to use a roller ( aluminium type) and a lot of elbow grease - apparently this is called consolidation. I have also seen videos of people dabbing down on the laminate with the bristles of a chopped of paint brush to burst the bubbles and force resin in. You'd need to work quickly to use this approach so maybe the roller is a better idea.

ii) To get a smooth finish on the board to replace the impression of the woven peel ply on the surface, I re-coated it with a small amount of resin and vac bagged it with the vac material directly on the surface. While this did leave the surface with more shine on it, surface was not uniform and had the impressions of wrinkles on it. Overall it was a disappointing finish. Next time I think I'll resign myself to needing to work the surface post curing. sanding, clearcoating and polishing. this seems unavoidable unless protective layer like PBT is being used. However, the PBT is expensive ( I've only come across a supplier in the US snowbaordingmaterials.com and its about $80 printed and delivered.) and apparently adds weight as it traps the resin.

iii) Reusing vac film and tacky tape - as occurred in the first board when I reused the vac film and tacky tape I got leaks that I couldn't close off. Finding the leaks is a real bastard. I had fairly rapid leaks at one point without the tell tail hissing sign that normally goes along with it. So, I had to just keep working around the edges and pushing it down. I have a feeling that threads of glass falling across the tape may have been responsible for keep small channels of air open. The only way to tell would be to have some really sensitive listening device. I'm going to see if I can get hold of a disused 'stethescope' or whether there might be Jaycar electronics kit for such a thing. I have a vague memory of seeing something like it.

Dean. Great minds think a like!! I just got hold of a stethescope last night as well. A mate of mine is a GP and apparently drug reps give them out as gifts. I scored a special viagra stethescope!!!!!Matt

Also, re pouring the rails. Yes again! That is my plan for the next board. I'm thinking of making the rails the full thickness of the board instead of 2 layers so I'm going to route through and keep the excess to make a mould with. A bit of masking tape on the underside to create the channel between the board and the excess material. For a part depth groove check this out

http://www.youtube.com/watch?v=d5NNDktWgn4

One guy on kiteform cautioned against putting chopped fibres in the resin as he said it leads to earlier fracturing in the resin but I can't really follow the logic. I'll be looking forward to seeing the photos when work starts.